Will set the current user, if the current user is not set. The current user
will be set to the logged-in person. If no user is logged-in, then it will
set the current user to 0, which is invalid and won't have any permissions.

This function is used by the pluggable functions wp_get_current_user() and
get_currentuserinfo(), the latter of which is deprecated but used for backward
compatibility.